1. Field of the Invention
This invention relates to heliostats which track the apparent motion of the sun to focus radiant solar energy upon a remote receiver, and more particularly provides apparatus for properly aiming the heliostat.
2. Description of the Prior Art
Increasing energy concerns have focused substantial attention upon new systems for harnessing solar energy. Typical of proposed central-station solar power plants are those which rely on large arrays of mirrors for concentration of solar energy upon a remote receiver. For example, in a 100 megawatt-electrical solar power plant the outermost mirrors can be 3,500 to 4,000 feet from the receiver and there can be about 40,000 mirror assemblies, each of 40 square meters, in the mirror field.
In order to obtain an efficient useful energy production with such systems, it is of maximum importance that each mirror assembly be properly oriented throughout the period of the sun's daily and seasonal availability to properly focus incident radiation upon the receiver. The accuracy of this tracking should be in the range of milliradians in both azimuthal and elevational directions in order to take maximum advantage of the sun's energy.
Two basic methods for heliostat guidance exist. The first is a time-oriented clock-controlled drive system. Such systems cannot provide the required accuracy for each individual mirror assembly, particularly when it is recognized that environmental effects such as ground settling about the mirrors and the receiver, and wind loading particularly of the receiver, typically several hundred feet high, are not adjusted by a timed system. The second method, which can be combined with a timed system, involves some means for sensing the positions of the sun and the receiver or target, and adjusting the position of the heliostat to best focus the sun's radiation from the mirror to the receiver. An example of this type of system is provided in U.S. Pat. No. 4,013,885 which provides a mirror with a central aperture or lens through which radiation from the sun and infrared radiation from the receiver pass. Through use of a light dispersive hemisphere, a second back mirror and other components, images of the sun and receiver are formed on a detector and the mirror is aimed by aligning these images such that they are coincident on the detector. While the system offers substantial advantages and accuracy, it presents some characteristics which can be improved upon such as numerous components and reliance upon infrared energy from the receiver which may not always be available, such as under cloud cover or initial daily start-up. Further, it is unclear how the system would respond when the sun, receiver and mirror aperture are all aligned, as well as the effect of the high energy radiation and corresponding temperatures upon the reliability of various system components.
Accordingly, it is desirable to provide a heliostat aiming system which provides the required accuracy with a minimum amount of components, which is of a high reliability, and which maximizes the reflected solar radiation during varying sun positions and environmental conditions.